The long term goal of this research projjct is to define, in molecular terms, general paradigms for the regulation of gene expression in bacteria. We are studying the regulation of the histidine operon of Salmonella typhimurium, and the regulation of the isoleucine and valine operons in Escherichia coli K12. The histidine regulation project is divided into two sub-prrjects: the origin and replication of the pihistidine plasmid and the role of PRPP-ATP ribosyl transferase in histidine regulation. The isoleucine and valine regulation project consists of examining the role of threonine deaminase in the regulatory mechanism and the identification of other regulatory elements both internal and external to the structural genes which act either in cis or in trans.